This invention relates to a highly purified biogenic silica product obtained from diatomite and methods for preparing same. More particularly, this invention pertains to a highly purified biogenic silica product which bears the distinguishing porous and intricate structure of silica unique to diatomite and which has an extraordinarily high silica (i.e., silicon dioxide, SiO2) content and low density, resulting in a high silica specific volume. The present invention also pertains to modified highly purified biogenic silica products having exceptionally low content of soluble metals and/or extreme brightness.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation; full citations for these documents may be found at the end of the specification immediately preceding the claims. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
The present invention relates to purified diatomite products which both retain the intricate and porous structure unique to diatomite (i.e., which have an intricate and porous diatomaceous silica structure), and have a unique combination of special physical and chemical properties (such as, for example, high purity, low density, low soluble impurity content, low total impurity content, and high brightness). The unique combination of special properties offered by the highly purified diatomite products of the present invention (which permit, inter alia, improved efficiency and economy) are not offered by other known diatomite products.
Currently, diatomite products are used in a wide variety of applications, including, but not limited to, separation, adsorption, support, and functional filler applications. Recent reviews (Breese, 1994; Engh, 1994) provide particularly useful introductions to the properties and uses of diatomite. The purified diatomite products of the present invention have both the intricate and porous structure unique to diatomite silica and exceptional purity, and therefore permit greatly increased effectiveness in many of these applications.
Diatomite products are obtained from diatomaceous earth (also known as kieselguhr), which is a sediment enriched in biogenic silica (i.e., silica produced or brought about by living organisms) in the form of the siliceous frustules (i.e., shells or skeletons) of diatoms. Diatoms are a diverse array of microscopic, single-celled golden brown algae of the class Bacillariophyceae, which possess an ornate siliceous skeleton (i.e., frustule) of varied and intricate structure consisting of two valves which, in the living diatom, fit together much like a pill box. The morphology of the frustules varies widely among species and serves as the basis for taxonomic classification; over at least 2,000 distinct species are known. The surface of each valve is punctuated by a series of openings that comprise the complex fine structure of the frustule and impart a design that is distinctive to individual species. The size of typical frustules ranges from 0.75 to 1,000 xcexcm, although the majority are in the range of 10 to 150 xcexcm. These frustules are sufficiently durable to retain much of their porous and intricate structure virtually intact through long periods of geologic time when preserved in conditions that maintain chemical equilibrium.
The fundamental chemical composition and the intricate and porous structure of the diatom frustule give diatomite unique commercial value and versatility unmatched by other natural forms of silica in, for example, filtration and filler applications. The fine particulate structure of the diatom skeleton imparts low density and high surface area, as well as high porosity and permeability.
Diatomite products may be manufactured by a variety of methods and from numerous resources, offering diversity in both physical and chemical characteristics.
In the field of filtration, many methods of particle separation from fluids employ diatomite products as filter aids. The intricate and porous structure unique to diatomite silica is particularly effective for the physical entrapment of particles in filtration processes. It is common practice to employ diatomite products when improving the clarity of fluids that contain suspended particles or particulate matter, or have turbidity.
Diatomite products are often applied to a septum to improve clarity and increase flow rate in filtration processes, in a step sometimes referred to as xe2x80x9cprecoating.xe2x80x9d Diatomite is also often added directly to a fluid as it is being filtered to reduce the loading of undesirable particulate at the septum while maintaining a designed liquid flow rate, in a step often referred to as xe2x80x9cbody feeding.xe2x80x9d Depending on the particular separation involved, diatomite products may be used in precoating; body feeding, or both. The principles involved with diatomite filtration have been reviewed (Kiefer, 1991).
In some filtration applications, different diatomite products are blended together to further modify or optimize the filtration process. Also, diatomite products are sometimes combined with other substances. In some cases, these combinations may involve simple mixtures, for example, with cellulose, activated charcoal, clay, or other materials. In other cases, these combinations are composites in which diatomite products are intimately compounded with other ingredients to make sheets, pads, or cartridges. Still more elaborate modifications of any of these diatomite products are used for filtration or separation, involving, for example, surface treatment and the addition of chemicals to diatomite products, mixtures, or their composites.
In certain circumstances, diatomite products may also exhibit unique adsorptive properties during filtration that can greatly enhance clarification or purification of a fluid. These adsorptive properties are highly specific, and depend upon weak forces for attraction of the adsorbed species to weak electrical charges at the surface of diatomite, or upon the reactivity of silanol (i.e., xe2x89xa1Sixe2x80x94OH) functional groups that often occur at the diatomite surface. For example, an ionized silanol group (i.e., xe2x89xa1Sixe2x80x94O) may react with a hydronium ion (i.e., H3O+) contributed by an acidic substance in solution, for example, citric acid (i.e., C6H8O7), adsorbing the donated H+ at the surface in the process.
The intricate and porous structure of silica unique to diatomite products also permits their commercial use to provide antiblock properties to polymers. Diatomite products are often used to alter the appearance or properties of paints, enamels, lacquers, and related coatings and finishes. In addition to their use in paper or cellulose-bearing filter media, diatomite products are used commercially in paper processing applications, and they are essential to the processing of certain commercial catalysts. Diatomite products are also used as chromatographic supports, and are especially suited to gas-liquid chromatographic methods.
Regardless of the manner in which a diatomite product is used in separating particles from fluids, the diatomite product must make contact with the fluid from which particles are being removed. As ingredients in polymers, plastics, paints, coatings, and other formulations, the diatomite product also comes into contact with most of the other ingredients in the formulation. For this reason, high silica purity and low solubility of impurities in the diatomite product are often highly desirable properties. In addition, the efficiency and usefulness of a diatomite product used as a filter aid is also related to its density when in contact with fluid during filtration operation. In most instances, a diatomite product with low density is highly desirable.
The highly purified diatomite products of the present invention, and their further modifications, are useful in numerous other applications. As the effectiveness of diatomite in its applications is generally related to the presence of the porous and intricate structure of silica unique to diatomite in combination with high silica purity, the highly purified diatomite products of the present invention offer these distinguishing characteristics in greater degree than heretofore possible.
One aspect of the present invention provides a highly purified diatomite product having an intricate and porous diatomaceous silica structure, said product selected from the group consisting of: a non-calcined product having a silica specific volume greater than 3.5; a calcined product having a silica specific volume greater than 3.6; and a flux calcined product having a silica specific volume greater than 3.3. In a preferred embodiment, the non-calcined product has a total silica content of greater than 95% (w/w) SiO2 on an ignited basis; the calcined product has a total silica content of greater than 98% (w/w) SiO2 on an ignited basis; and, the flux calcined product has a total silica content of greater than 92% (w/w) SiO2 on an ignited basis. In a preferred embodiment, the product has a beer-soluble aluminum content of less than 10 mg Al/kg product. In another preferred embodiment, the product has a beer-soluble iron content of less than 7 mg Fe/kg product.
Another aspect of the present invention provides a highly purified non-calcined diatomite product having an intricate and porous diatomaceous silica structure, said product having a silica specific volume greater than 3.5. In a preferred embodiment, the non-calcined product has a total silica content of greater than 95% (w/w) SiO2 on an ignited basis. In another preferred embodiment, the non-calcined product has a beer-soluble aluminum content of less than 10 mg Al/kg product. In yet another preferred embodiment, the non-calcined product has a beer-soluble iron content of less than 7 mg Fe/kg product. In still another preferred embodiment, the product has a specific resistance greater than 50 kxcexa9-cm.
Another aspect of the present invention provides a highly purified calcined diatomite product having an intricate and porous diatomaceous silica structure, said product having a silica specific volume greater than 3.6. In a preferred embodiment, the calcined product has a total silica content of greater than 98% (w/w) SiO2 on an ignited basis. In another preferred embodiment, the calcined product has a beer-soluble aluminum content of less than 10 mg Al/kg product. In yet another preferred embodiment, the calcined product has a beer-soluble iron content of less than 7 mg Fe/kg product. In still another preferred embodiment, the calcined product has a specific resistance greater than 80 kxcexa9-cm. In yet another preferred embodiment, the calcined product has a blue light reflected brightness of greater than 96%.
Another aspect of the present invention provides a highly purified flux calcined diatomite product having an intricate and porous diatomaceous silica structure, said product having a silica specific volume greater than 3.3. In a preferred embodiment, the flux calcined product has a total silica content of greater than 92% (w/w) SiO2 on an ignited basis. In another preferred embodiment, the flux calcined product has a beer-soluble aluminum content of less than 10 mg Al/kg product. In yet another preferred embodiment, the flux calcined product has a beer-soluble iron content of less than 7 mg Fe/kg product. In still another preferred embodiment, the flux calcined product has a blue light reflected brightness of greater than 95%. In yet another preferred embodiment, the flux calcined product has a combined total sodium (Na2O) content and total silica (SiO2) content of greater than 98% (w/w) on an ignited basis.
Another aspect of the present invention provides a highly purified diatomite product having an intricate and porous diatomaceous silica structure, said product having a blue light reflected brightness greater than 99%.